Electron discharge device



Oct. 19, 1965 M. v. DUERR 3,213,311

ELECTRON DISCHARGE DEVICE Filed April 15, 1962 2 SheetsSheet 1 Fig. 2

WITNESSES INVENTOR Melvin Duerr j ml g fi V ATfORNEY United States Fatent O 3,213,311 ELECTRON DKSCHARGE DEVICE Melvin V. Duel-r, Elmira, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 13, 1962, Ser. No. 187,257 7 Claims. (Cl. 313-82) This invention relates to electron discharge devices and more particularly to a cathode ray tube of the type employing electrostatic means for forming and focusing an electron beam onto a target.

In television receivers, and other electronic devices in which information is displayed upon a luminescent screen by exciting the screen with an electron beam, it is highly desirable to attain maximum modulation of the light output of the cathode ray tube in response to a given change of amplitude of the control signal applied to the device. If one is able to provide an alectron gun which provides a relatively large change in beam current in response to a relatively small input signal variation, then it is possible to simplify the circuitry of the television set. It is possible to eliminate a video amplifier and in addition to simplify other circuitry of the television set and particularly in the power supply of the television receiver. Stated in another way, the cathode ray tube should have a high transconductance while maintaining brightness and resolution suitable for normal television viewing.

It is presently known in the art that for a given drive voltage, increased cathode and beam currents may be achieved by modulating the cathode of the electron gun rather than the control grid which is normally that grid adjacent the cathode and referred to as the first grid or (3 In the use of the electron gun in which cathode modulation is used, the attainment of minimum drive voltages and high transconductance is dependent upon a gun in which the potential of the second electrode or grid from the cathode and referred to as G is less than 100 volts positive when G is held at ground. The G electrode prior to this low G type design normally operated at about 300 volts positive.

Guns designed for lower G voltages have higher transconductances. As the t-ransconductance of the gun increases, the less signal or drive voltage is required.

The conventional electron gun includes third and fifth grids or electrodes G and G which operate at a high positive potential of about 16,000 volts and an intermediate fourth grid or electrode 6., operating at a potential of about 300 volts. The third, fourth and fifth grids provide the principal focusing lens of the electron gun. It is therefore important in the low G type of gun that the penetration of the high voltages from the G electrode and those subsequent do not penetrate into the cathode to any extent since it would then require a higher drive voltage to out the tube off. Electron guns in which the high voltages of the G and subsequent electrodes do not penetrate into the cathode area are referred to as low G penetration guns.

On difficulty that is found in present cathode ray tubes and primarily believed due to the electrostatic gun design is a defect known as spot blooming. This phenomena manifests itself as a considerable increase in the spot size on the luminescent screen or even a holo when the control grid potential approaches zero. This causes severe defocusing in the highlight portions of the television picture.

It has been found that this undesirable aberration or defocusing in the electron gun can be largely eliminated by suitably adjusting the strength of the lens between the G and G electrodes of the electron gun. The exact mechanism of this process is not entirely clear but the following explanation is believed to be correct in substance.

3,213,311 Patented Get. 19, 1965 As in cathode drive, the cathode approaches zero voltage, the beam angle from the triode comprised of the cathode, G and G becomes large. Furthermore, at zero cathode voltage, electrons may strike the side of the G electrode which is connected to ground or zero voltage. These electrons, on striking the G electrode are reflected therefrom and enter the main or principal lens of the electron gun at rather large divergent angles and are not focused in the same image plane as the desired electrons from the center of the emitting cathode. The result, of course, is that a halo is found around the main spot on the luminiscent screen. This halo as previously indicated is undesirable. It has been found that by suitably shaping the lens between the G and G electrodes of the electron g-un and especially by bringing the apertures in these two electrodes near to the electron beam, one is able to correct and prevent the marginal electrons from diverging from the beam axis to the extent that they effect this lens aberration. It is found that by the use of an intrusion-type G electrode one is able to obtain a good beam spot over prior art designs.

In addition, one would expect that by providing an intrusion-type G electrode that an increased penetration of the G field through the G aperture would result in the cutoff being considerably dependent upon the G potential. This would of course result in a reduction of the modulation sensitivity of the electron gun A reduction in modulation sensitivity would result if G penetration were allowed to become too great. This invention describes an arrangement of electrodes which controls the amount of G penetration when the lens between G and G is adjusted for good beam spot so as not to destroy the good modulation sensitivity.

The penetration of the G electrode and subsequent electrodes into the cathode of prior art designs is of the low penetration type and the G electrode is the electrode which substantially defines the cutolf voltage. Hence, to secure the improved modulation sensitivity arising from cathode modulation, it is essential to operate G at a low potential that is of the order of volts or less. Here tofore, it has been generally thought that in such low G voltage guns some penetration from G was necessary in order to prevent soft focus tubes and that the requirement of good cathode modulation sensitivity by minimizing high voltage penetration and the attainment of good focus conditions were contradictory. It has been found, that these two factors do not necessarily oppose each other and particularly so in the electron gun described herein. It has been found that with the device defined herein that one is able to simultaneously achieve small high voltage penetration with consequently high cathode modulation sensitivity while at the same time using a powerful intrusion-type lens between G and G to hold the beam together and minimize spot blooming at high drive conditions.

It is accordingly an object of this invention to provide an improved cathode ray tube.

It is another to provide an improved electron gun of high transconductance.

It is another object to provide an improved electron gun which provides high transconductance and high beam current.

It is another object to provide an improved electron gun which provides a small area beam spot on the luminescent screen of a cathode ray tube and is substantially uniform in intensity over its area.

Briefly, the present invention accomplishes the abovecited objects by providing an electron gun having improved modulation sensitivity with cathode modulation and a structure providing high G penetration. This high penetration factor is attained by optimum design of the spacing between the cathode G and G and in addition,

the size of the apertures in G and G In addition, improved spot characteristics of the electron gun as well as higher beam current is obtained by providing an intrusion part in the G electrode and by providing optimum spacing between G and G as well as the apertures therein.

Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterized invention will be pointed out in particularity in the clams annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIGURE 1 is a plan view partially in section of a cathode ray tube embodying the teaching of this invention;

FIG. 2 is an enlarged sectional view of a portion of the neck of the tube shown in FIG. 1 and illustrating the electron gun in accordance with the teachings of this invention; and,

FIGS. 3, 4 and 5 are graphical representations of operating characteristics of the devices shown in FIGS. 1 and 2 in comparison with like characteristics of the prior art design.

Referring to FIGS. 1 and 2, a cathode ray tube 11 is shown comprising an evacuated envelope 13 enclosing an electron gun 20 spaced from a fluorescent image display screen 15. The electron gun 20 is provided within the neck portion 17 of the envelope 13. The electron gun 20 provides the source, control, acceleration and focusing of an electron beam generated by the gun 20 which is directed toward the screen and is deflected by any suitable deflection means such as electromagnetic deflection yoke 21. The screen 15 as well as the final electrode of the electron gun are normally maintained at a potential of about 14 to 18 kilovolts depending upon the particular application. This high potential accelerates the electrons to a high velocity and they impinge upon the screen 15 to excite the phosphor in screen 15 and cause emission of light therefrom. The high voltage applied to the screen 15 and to the high voltage electrodes of the electron gun 20 is normally applied to an anode button 23. An internal electrically conductive coating on the flared portion of the envelope 13 extends back into the neck portion 17 to provide the necessary voltage for the high voltage electrodes of the electron gun 20. This potential is connected by means of snubbers 27 contacting the conductive coating 25 and electrically connected to the high voltage electrodes. The voltages for the remaining electrodes of the electron gun are normally provided by suitable lead-in-members through a base portion 29 of the envelope 13.

The electron gun 20 which is shown in more detail in FIG. 2 comprises a series of electrodes which are supported in alignment with each other in a conventional manner by means of insulating support rod members 31 extending longitudinally of the neck portion 17. Suitable stud members 18 are secured to the electrodes and imbedded in the rods 31. The electrodes within the electron gun 20 are normally aligned with the longitudinal axis of the cathode ray tube.

The electron gun 20 includes a tubular indirectly heated cathode 33 supported by an annular ceramic member 35 within a control or first grid G 37 of the gun 20. A filament or a heater 39 of any Well known type is contained Within the tubular cathode 33 for heating the cathode 33. The end tubular cathode 33 facing the screen 15 is closed and has an electron emissive coating 41. provided on the exterior surface thereof.

The control grid 37 is a cup-shaped member which faces rearwardly that is, whose open end faces away from the screen 15. The grid 37 consists of a tubular portion 38 with a diaphragm 40 having a centrally located aperture 42 overlying the emissive coating 41 on the cathode 33. The diameter of the aperture 42 in the diaphragm or plate 40 is about .036 inch and is referred to in the drawing as al The plate 40 has central portion 43 surrounding the aperture 42 that has a concave configuration facing away from the cathode 33. The object of the concave portion 43 is to provide a reduction in thickness of the diaphragm 40 near the boundary of the aperture 42 in relation to the overall thickness of the diaphragm 40 itself to obtain a minimum spacing between the surface of the emissive layer 41 and the surface of the diaphragm 40 adjacent the aperture 42 on the side of the diaphragm 40 facing the screen 15. The distance between the surface of the coating 41 and the surface of the diaphragm 40 of the control grid 37 is designated as S in the FIG. 2 and this distance is about .004 inch. The thickness of the diaphragm 40 in G at the boundary of the aperture 42 is about .004 inch.

Closely spaced from the grid electrode 37 is the second grid G 50. The electrode 50 is also a metal cup-like member with the open end thereof facing the screen 15. The electrode 50 includes a tubular portion 51 and a diaphragm 52. The diaphragm 52 of the electrode 50 has a centrally located aperture 53 aligned with the aperture 42 in G A central portion 54 of the diaphragm 52 surrounding the aperture 53 is deformed or depressed to provide concave or inverted frusto-conical portion facing the screen 15. The aperture 53 is provided substantially at the center of the concave portion 54 and the diameter of the aperture 53 is about .028 inch and is hereinafter referred to as d The spacing between the boundary surface of the aperture 53 facing the coating 41 and the surface of the electrode 37 adjacent the aperture 42 facing the screen 15 is about .004 inch and is hereinafter referred to as S The electrode 50 includes an additional cup-shaped element or shield 57 which is telescoped into the tubular portion 51 with the open end facing the screen 15. The element 57 includes a tubular portion 47 which fits Within the tubular portion 51 and a diaphragm or plate 58 closing the end of the portion 47 adjacent the diaphragm 52. The diaphragm portion or plate 58 is in contact with the surface of the diaphragm 52 facing the screen 15 and is electrically and mechanically connected to electrode 50. The plate or diaphragm member 58 of shield 57 has a centrally located aperture 59 which is hereinafter referred to as d The spacing between the surface of the concave portion 54 of electrode 50 adjacent the aperture 53 and facing the screen 15 and the surface of the plate member 58 facing the cathode 33 is about .010 inch and is hereinafter referred to as S It should be noted here that the spacing between electrode 37 and electrode 50 at the outer periphery of the two members is about .010 inch and hereinafter referred to as S The outer diameter of electrodes 37 and 50 is about .5 inch.

The structure described above provides a triode assem-- bly in which the penetration of electrode 50 or G is high. The maximum amount of penetration within this structure is limited primarily by the practical electrode spacing in order to prevent arcing and other associated problems. In general, it is desirable to use the cathode to G spacing S to obtain the desired cutoff. In this structure S similar to S is a means by which G penetration is increased. Because of this high penetration factor, the cathode cutoff voltage is largely determined by the voltage on G which normally is 20 to 50 volts while the voltage on G is zero. Limitations imposed on the minimum aperture spacing are that they be consistent with manufacturing standards for avoiding arcing over between electrodes and changes in spacing due to temperature variations. The extended aperture 53 on G allows the actual aperture spacing S to be held to a very small value and yet maintain a relatively large electrode spacing S to prevent shorting. To obtain the high G penetration and high transconductance, S and S must not exceed .17d Limitations imposed on the extended grid aperture d are based on the fact that although the G penetration factor D is increased as a result of a small aperture, beam current interception and a corresponding decrease in transconductance will determine a minimum Value. Also a significant reduction in the G penetration factor D will result as this aperture diameter approaches or is made larger than that of G The relatiOnship that (1 al be less than .9 and greater than .7 was found to give the most favorable or optimum performance. The penetration factor D is equal to V /V D is defined as the G2 penetration factor where the expression V is the potential difference between the cathode and G and V is the negative voltage required on the G to turn the beam off with the final anode or G and G is set to zero potential. The penetration factor or D of G in the present design is approximately 5.

The next electrode adjacent to electrode 50 or G is electrode 60 or G The electrode 60 consists of an outer tubular member 61 having the end thereof facing the screen closed by a diaphragm 62 with a centrally located aperture 63 therein. The aperture 63 has a diameter of about .090 inch and is referred to as d The outer member 61 of G consists of a first end portion 64 which is of substantially the same diameter as th tubular portion 51 and a second end portion 65 of reduced diameter and a portion connecting portions 64 and 65. An inner tubular member 67 is provided within the outer member 61 and consists of a tubular portion 68 having an outer diameter substantially equal to the inner diameter of the reduced portion 65 of the outer member 61 so that it may be telescoped within the outer member 61. The remaining portion of the inner member 67 is a frusto-conical portion 69 with the smaller portion of the conical portion 69 facing the second grid 56. The frusto-conical portion 69 actually extends or protrudes beyond the outer member 61 with respect to the second grid 50. This frusto-conical member 69 is provided with an entrance aperture or opening 70 facing the second grid 50. The boundary of the aperture 70 is spaced at a distance of about .170 inch from plate 57 hereinafter referred to as S The diameter of the entrance aperture 70 is about .120 inch and is hereinafter referred to as (1 By adjusting the electrode spacing S and the size of the aperture 70 in the intrusion part 69, considerable improvement in spot size characteristics have been obtained. The maximum gain or improvement that can be obtained is of course limited by how much high voltage penetration can be permitted before it largely determines the cutoff of the electron gun. Obviously, with the high modulation sensitivity attained with a triode structure as previously described, excessive high voltage penetration from G is highly undesirable. The shield member 57 within G is one means of restricting or determining the amount of high voltage penetration from G Actually the plate member 57 in G and the other portions of G act as a shield to separate the triode section comprising the cathode 33, first grid 37, and concave portion 54 of the G from the effects of the potential of G The diameter of the aperture 59 or d of the shield 57 is largely a compromise as a result of (l) a gain in the G penetration factor D (2) high voltage penetration, and (3) the prefocus lens strength. As the size of the aperture [i is made smaller, a slight increase in the G penetration D and a considerable reduction in high voltage penetration D are experienced. The G penetration factor D is equal to V V and is about .0075. V is the potential difference between the cathode and G and G and V is the negative voltage required on G to turn the beam off with voltage on G at zero. Because of the reduction in high voltage penetration D the spot size could be expected to increase at high beam currents. The prefocus lens between G and G is adjusted to overcome this effect. The limitation imposed on the size of the aperture ri in G is that in an attempt to increase the high voltage penetration by adjusting the spacing S the resulting increase in prefocus lens strength will eventually result in an increase in spot size at the screen. The diameter of the aperture d in the shield 57 therefore, must be within a range to give maximum G penetration D and yet allow enough high voltage penetration to control spot growth and blooming without unduly increasing the strength of the prefocus lens. This objective is attained by making the aperture d in the shield 57 approximately equal to the size of the aperture al in the first grid 37 and the spacing S between the apertures 0' and d not greater than .6d Also the aperture 11.; or 70 must be smaller than the diameter of the outer tubular portion 51 or G and the grid spacing S must be equal to or greater than 3d A tubular electrode or G is positioned adjacent to electrode 60. A tubular electrode having a diaphragm 82 with an aperture 84 is positioned adjacent electrode 75. The diameter of the aperture is about .150 inch.

The following data may be considered as an example of suitable dimensions and voltages for an electron gun in accordance with the invention.

Diameter:

d =.036 inch Spacing S =.004 inch d =.028 inch Spacing S =.004 inch d =.036 inch Spacing S =.010 inch d =.l20 inch Spacing S =.0l0 inch d =.090 inch Spacing S =.170 inch d =.150 inch The curves in FIGS. 3, 4 and 5 show the performance obtained from a gun constructed according to this invention as compared to a conventional low G gun. Curves A are for a conventional or prior art tube while curves B are for the electron gun described herein. This data shows that substanital increase in transductance is obtained giving the tube considerable drive advantage over the conventional type and this improvement was obtained with a considerable improvement in spot size. This improvement in transconductance is clearly shown in FIG. 3. FIG. 4 also illustrates the increased transconductance and in addition clearly illustrates that the maximum beam current obtainable from the tube is substantially increased over the prior art type of device. FIG. 5 also shows a comparison between the prior art and the invention described herein and illustrates the improvement in focus or spot size on the display screen. FIG. 5 also indicates the increased amount of current obtainable from the electron gun described herein in comparison to the prior art device.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to this specific arrangement shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. An electron discharge device comprising an image screen, an electron gun formed to provide the source, acceleration, control and focusing of an electron beam spac d from said screen, said electron gun comprising a cathode, a first grid, a second grid and a third grid arranged in the direction of electron travel towards said screen, said first grid comprising a cylinder having an end wall formed to provide a central region having a concave surface facing said second grid and an aperture provided in said central region aligned with said cathode, the wall surrounding said aperture being of reduced thickness as compared with the remaining end wall, a second grid comprising a cylinder and having an end wall on the end of said cylinder facing said first grid, the end wall of said second grid formed to provide an aperture aligned with the aperture in said first grid, the central portion of said end wall of said second grid surrounding the aperture deformed to provide a frusto-conical portion with the smaller portion forming the boundary of the aperture closer to said first grid than the remaining portion of the end wall of said second grid, the ratio of the diameter of'the aperture in said second grid to the diameter of the aperture in said first grid being greater than .7 and less than .9, the spacing between said cathode and said first grid not exceeding .17 times the diameter of the aperture in said first grid and the spacing between said first grid and said second grid adjacent the apertures therein not exceeding .17 times the diameter of the aperture in said first grid, a shield member provided within the cylinder of said second grid and formed to provide an aperture aligned with said aperture in said second grid, said shield member electrically and mechanically connected to said end wall of said second grid with the aperture in said second grid electrode and the aperture in said shield spaced apart by a distance no greater than .6 of the diameter of the aperture in the shield, the diameter of the aperture in said shield member being about equal to the diameter of the aperture in said first grid, said third grid comprising a tubular member having the end thereof facing said second grid provided with a frusto-conical portion with the opening in the frusto-conical portion aligned with the aperture in said second grid, the diameter of the entrance opening in the frusto-conical portion of said third grid being less than the cylindrical portion of said second grid and the spacing between the aperture in said shield member and the aperture in said third grid being greater than three times the diameter of the aperture in said shield member.

2. An electron discharge device comprising an image screen, an electron gun for generating and forming an electron beam which is directed onto said image screen, :said electron gun comprising a cathode, a first grid, a second grid and a third grid arranged in the direction of electron travel towards said screen, said first grid comprising a tubular member having an end Wall transverse to said electron beam and formed to provide a central region having a concave surface facing said second grid and an aperture provided in said central region aligned with said cathode, a second grid comprising a tubular member and having an end wall transverse to said electron beam and on the end of said tubular member facing said first grid, the end wall of said second grid formed to provide an aperture aligned with the aperture in said first grid, the central portion of said end Wall of said second grid surrounding the aperture deformed to provide a frusto-conical portion with the smaller portion of said frusto-conical portion forming the boundary of the aperture closer to said first grid than the remaining portion of the end wall of said second grid, the ratio of the diameter of the aperture in said second grid to the diameter of the aperture in said first grid being greater than .7 and less than .9, a shield member provided within the tubular portion of said second grid formed to provide an ape-rture aligned with said aperture in said second grid, said shield member electrically andme'chanically connected to said end wall of said second grid with the aperture in said second grid electrode and the aperture in said shield spaced apart by a distance less than .6 of the diameter of the aperture in said shield, the diameter of the aperture in said shield member being about equal to the diameter of the aperture in said first grid, said third grid comprising a tubular member having the end thereof facing said second grid provided with a frusto-conical portion With the opening in the frusto-conical portion aligned with the aperture in said second grid, the diameter of the entrance opening in the frusto-conical portion of said third grid being less than the diameter of said tubular portion of said second grid and the spacing between the aperture in said shield member and the aperture in said third grid being greater than three times the diameter of the aperture in id shi d membe w 3. Anelectron discharge device comprising an image screen, an electron gun for generating, forming and directing an electron beam onto said screen, said electron gun comprising a cathode, a first grid, a second grid and a third grid arranged in the direction of electron travel towards said screen, and first grid comprising a cylindrical member having an end wall in the end facing said grid and formed to provide a central region having a concave surface facing said second grid and an aperture providing in said central region aligned with said cathode, the end wall in said first grid surrounding said aperture being of reduced thickness as compared with the remaining portion of said end wall, the spacing between the surface of the end wall of said first grid facing said cathode and the emissive surface of said cathode less than .17 times the diameter of the aperture in the end wall of said first grid, a second grid comprising a cylindrical member having a diaphragm therein, the diaphragm of said second grid formed to provide an aperture aligned with the aperture in said first grid, the central portion of said diaphragm of said second grid surrounding the aperture deformed to provide a tubular portion having a smaller diameter portion forming the boundry of the aperture closer to said first grid than the remaining portion of said diaphragm of said second grid, the diameter of the aperture in the diaphragm of said second grid being less than the diameter of the aperture in the end wall of said first grid, the spacing between said first grid and said second grid adjacent the apertures therein less than .17 times the diameter of the aperture in said first grid, a shield member provided within the cylindrical member of said second grid formed to provide an aperture aligned with said aperture in said second grid, said shield member electrically and mechanically connected to said diaphragm of said second grid with the aperture in said second grid electrode and the aperture in said shield spaced apart by a distance less than .6 of the diameter of the aperture in the shield, the diameter of the aperture in said shield member of about the same diameter as the aperture in said first grid, said third grid comprising an outer cylindrical and an inner tubular member, said tubular member having the end thereof facing said second grid of smaller diameter than the remaining portion and providing an opening aligned with the aperture in said second grid, the diameter of the entrance opening in said inner tubular member of said third grid being less than the diameter of said cylindrical member of said second grid and the spacing between the aperture in said shield member and the aperture in said third grid being greater than three times the diameter of the aperture .in said shield member.

4. An electron discharge device comprising an electron gun and target, said electron gun comprising an electron emissive source, a first grid, a second grid and a third grid arranged in the direction of electron travel towards said target, said first grid comprising a tubular member having an end wall formed to provide an aperture aligned with said electron source, the wall surrounding said aperture being of reduced thickness as compared with the remaining portion of said end wall, a second grid comprising a tubular member and having an end wall on the end of said tubular member facing said first grid, the central portion of said end wall of said second grid formed to provide an aperture aligned with the aperture in said first grid and closer to the first grid than the remaining portion of the end wall of said second grid, the diameter of the aperture in said second grid being less than the diameter of the aperture in said first grid, a shield member provided within the tubular portion of said second grid electrode having a diaphragm with an aperture therein and aligned with said aperture in said second grid, said shield member electrically connected to said end wall of said second grid with the aperture in said second grid and said shield spaced apart by a distance less than .6 0f the diameter of the aperture in said shield, the diameter of the aperture in said shield member being greater than the diameter of the aperture in said second grid, said third electrode comprising an outer tubular member and an inner tubular member, said inner tubular member tapered to provide a smaller entrance aperture adjacent said second grid and aligned with the aperture in said second grid, the diameter of the entrance aperture of said third grid being less than the diameter of the tubular portion of said second grid and the spacing between the aperture in the shield member and the aperture in said third grid electrode greater than three times the diameter of the aperture in said shield member.

5. An electron discharge device comprising an elec tron gun and target, said electron gun comprising an electron emissive source, a first grid, a second grid and a third grid arranged in the direction of electron travel towards said target, said third grid being at a substantially higher potential than said first or second grids, said first grid including a diaphragm having an aperture therein and aligned with said electron source, a second grid including a diaphragm, the central portion of said diaphragm of said second grid formed to provide an aperture aligned with the aperture in said first grid and spaced closer to said first grid than the remaining portion of the diaphragm of said second grid, the diameter of the aperture in said second grid being less than the diameter of the aperture in said first grid, a shield member including a diaphragm positioned on the side of said second grid facing said third grid and having an aperture therein and aligned with said aperture in said second grid, said shield member electrically connected to said second grid with the aperture in said second grid and the aperture in said shield spaced apart by a distance less than the diameter of the aperture in said shield member, the diameter of the aperture in said shield member being greater than the diameter of the aperture in said second grid, said third electrode including a tubular member, said tubular member tapered to provide a smaller entrance aperture adjacent said shield member and aligned with the aperture in said shield member, the diameter of the entrance aperture of said third grid being about three times the diameter of the aperture in said shield member.

6. An electron discharge device comprising an electron gun and target, said electron gun comprising an electron emissive source, a first grid, a second grid and a third grid arranged in the direction of electron travel towards said target, said first grid comprising a tubular member having an end wall formed to provide an aperture aligned with said electron source, the wall surrounding said aperture being of reduced thickness as compared with the remaining portion of said end wall, a second grid comprising a tubular member of the same diameter as the tubular member in said first grid and having an end wall on the end of said tubular member facing said first grid, the central portion of said end wall of said second grid formed to provide an aperture aligned with the aperture in said first grid and closer to the end wall of said first grid than the remaining portion of the end wall of said second grid, the diameter of the aperture in said second grid being less than the diameter of the aperture in said first grid, a shield member provided within the tubular portion of said second grid electrode having a diaphragm with an aperture therein and aligned with said aperture in said second grid, said shield member electrically and mechanically connected to said end wall of said second grid with the aperture in said second grid and said shield spaced apart by a distance less than .6 of the diameter of the aperture in said shield, the diameter of the aperture in said shield member being greater than the diameter of the aperture in said second grid, said third electrode comprising an outer tubular member of the same diameter as the tubular member in said second grid and an inner tubular member, said inner tubular member of less diameter than said outer tubular member tapered to provide a smaller entrance aperture adjacent said shield and aligned with the aperture in said second grid, the diameter of the entrance aperture of said third grid being less than the diameter of the outer tubular member and spaced closer to said shield than said outer tubular portion and the spacing between the aperture in the shield member and the aperture in said third grid electrode greater than three times the diameter of the aperture in said shield member.

7. An electron discharge device comprising an electron gun and a target, said electron gun comprising an electron with a source, a first grid, a second grid and a third grid arranged in the direction of the electron travel to said target, said second grid operating at a potential of less than volts positive with respect to said electron emissive source, said third grid operating at a potential of several thousand volts positive with respect to said electron emissive source, said first grid including a diaphragm having an aperture therein and aligned with said electron source, a second grid including a diaphragm with a central portion of said diaphragm of said second grid formed to provide an aperture aligned with the aperture in said first grid and spaced closer to said first grid and the remaining portion of the diaphragm of said second grid, the diameter of the aperture in said second grid being less than the diameter of the aperture in said first grid, a shield member including a diaphragm positioned on the side of said second grid facing said third grid and having an aperture therein aligned with said aperture in said second grid, said shield member electrically connected to said second grid with the aperture in said second grid and the aperture in said shield spaced apart by a distance less than the diameter of the aperture in said shield member, the diameter of the aperture in said shield member being greater than the diameter of the aperture in said second grid, said third electrode including a tubular member, said tubular member tapered to provide a smaller entrance aperture adjacent said shield member and aligned with the aperture in said shield member, the diameter of the entrance aperture of said third grid being about three times the diameter of the aperture in said shield member.

References Cited by the Examiner UNITED STATES PATENTS 2,935,636 5/60 Knechtli 31382.1 3,008,064 11/61 Niklas 313-821 3,027,479 3/62 Benway 31382.1

DAVID J. GALVIN, Primary Examiner, 

4. AN ELECTRON DISCHARGE DEVICE COMPRISING AN ELECTRON GUN AND TARGET, SAID ELECTRON GUN COMPRISING AN ELECTRON EMISSIVE SOURCE, A FIRST GRID, A SECOND GRID AND A THIRD GRID ARRANGED IN THE DIRECTION OF ELECTRON TRAVEL TOWARDS SAID TARGET, SAID FIRST GRID COMPRISING A TUBULAR MEMBER HAVING AN END WALL FORMED TO PROVIDE AN APERTURE ALIGNED WITH SAID ELECTRON SOURCE, THE WALL SURROUNDING SAID APERTURE BEING OF REDUCED THICKNESS AS COMPARED WITH THE REMAINING PORTION OF SAID END WALL, A SECOND GRID COMPRISING A TUBULAR MEMBER AND HAVING AN END WALL ON THE END OF SAID TUBULAR MEMBER FACING SAID FIRST GRID, THE CENTRAL PORTION OF SAID END WALL OF SAID SECOND GRID FORMED TO PROVIDE AN APERTURE ALIGNED WITH THE APERTURE IN SAID FIRST GRID AND CLOSER TO THE FIRST GRID THAN THE REMAINING PORTION OF THE END WALL OF SAID SECOND GRID, THE DIAMETER OF THE APERTURE IN SAID SECOND GRID BEING LESS THAN THE DIAMETER OF THE APERTURE IN SAID FIRST GRID, A SHIELD MEMBER PROVIDED WITHIN THE TUBULAR PORTION OF SAID SECOND GRID ELECTRODE HAVING A DIAPHRAGM WITH AN APERTURE THEREIN AND ALIGNED WITH SAID APERTURE IN SAID SECOND GRID, SAID SHIELD MEMBER ELECTRICALLY CONNECTED TO SAID END WALL OF SAID SECOND GRID WITH THE APERTURE IN SAID SECOND GRID AND SAID SHIELD SPACED APART BY A DISTANCE LESS THAN .6 OF THE DIAMETER OF THE APERTURE IN SAID SHIELD, THE DIAMETER OF THE APERTURE IN SAID SHIELD MEMBER BEING GREATER THAN THE DIAMETER OF THE APERTURE IN SAID SECOND GRID, SAID THIRD ELECTRODE COMPRISING AN OUTER TUBULAR MEMBER AND AN INNER TUBULAR MEMBER, SAID INNER TUBULAR MEMBER TAPERED TO PROVIDE A SMALLER ENTRANCE APERTURE ADJACENT SAID SECOND GRID AND ALIGNED WITH THE APERTURE OF SAID SECOND GRID, THE DIAMETER OF THE ENTRANCE APERTURE OF SAID THIRD GRID BEING LESS THAN THE DIAMETER OF THE TUBULAR PORTION OF SAID SECOND GRID AND THE SPACING BETWEEN THE APERTURE IN THE SHIELD MEMBER AND THE APERTURE IN SAID THIRD GRID ELECTRODE GREATER THAN THREE TIMES THE DIAMETER OF THE APERTURE IN SAID SHIELD MEMBER. 